US7345324B2

Light emitting diodes with graded composition active regions

Summary by NHIP

Graded III-Nitride LED

The light emitting device features an active region with a graded III-Nitride semiconductor alloy layer overlying a first semiconductor layer. This alloy, such as In x Al y Ga 1-x-y N, exhibits a composition graded asymmetrically or linearly to reduce piezoelectric field effects within the wurtzite crystal structure.

Claim Score by NHIP

Read claim 1, the broadest

Abstract

A light emitting device in accordance with an embodiment of the present invention includes a first semiconductor layer of a first conductivity type having a first surface, and an active region formed overlying the first semiconductor layer. The active region includes a second semiconductor layer which is either a quantum well layer or a barrier layer. The second semiconductor layer is formed from a semiconductor alloy having a composition graded in a direction substantially perpendicular to the first surface of the first semiconductor layer. The light emitting device also includes a third semiconductor layer of a second conductivity type formed overlying the active region.

US7345324B2, drawing sheet 1 — Sheet 1 of 7

Term

Term ended

Expired 12 October 2021, 5 years ago.

15 claims: 2 independent, 13 dependent

1
Broadest claimClaim Score 67, broad(NHIP)A light emitting device comprising:a first semiconductor layer of a first conductivity type having a first surface;an active region overlying the first semiconductor layer, the active region including a second semiconductor layer, the second semiconductor layer one of a quantum well layer and a barrier layer, the second semiconductor layer formed from a III-Nitride semiconductor alloy having a composition graded in a direction substantially perpendicular to the first surface of the first semiconductor layer;and a third semiconductor layer of a second conductivity type overlying the active region.
9
A light emitting device comprising:a first semiconductor layer of a first conductivity type having a first surface;an active region overlying the first semiconductor layer, the active region including a plurality of quantum well layers and at least one barrier layer, the barrier layer formed from a III-Nitride semiconductor alloy having an indium mole fraction graded in a direction substantially perpendicular to the first surface of the first semiconductor layer;and another semiconductor layer of a second conductivity type overlying the active region.

Independent claims2

49 paragraphs in 5 sections, as filed

0001This application is a division of application Ser. No. 09/912,589 filed Jul. 24, 2001 now U.S. Pat. No. 6,955,933 and incorporated herein by reference.

FIELD OF THE INVENTION

0002The present invention relates generally to semiconductor light emitting devices and more particularly to improving the light output of the active region in light emitting devices.

BACKGROUND

0003III-Nitride light emitting devices are based on semiconducting alloys of nitrogen with elements from group III of the periodic table. Examples of such III-Nitride devices include InxAlyGa1-x-yN light emitting diodes (LEDs) and laser diodes (LDs).

0004The active regions of InxAlyGa1-x-yN LEDs and LDs typically include one or more InxAlyGa1-x-yN quantum well and barrier layers. These layers typically have alloy compositions which differ from each other and differ from the surrounding layers in the device. As a consequence of these composition differences, the layers in the active region of an InxAlyGa1-x-yN light emitting device are typically biaxially strained. It should be noted that in the notation InxAlyGa1-x-yN, 0≦x≦1, 0≦y≦1, and x+y≦1.

0005InxAlyGa1-x-yN crystals such as those from which InxAlyGa1-x-yN light emitting devices are formed typically adopt either the wurtzite or the zinc blende crystal structure. Both of these crystal structures are piezoelectric. That is, both develop an internal electric field when stressed. In addition, the low symmetry of the wurtzite crystal structure produces a spontaneous polarization. As a result of their biaxial strain and of the piezoelectric nature of InxAlyGa1-x-yN, and as a result of the spontaneous polarization (when present), the quantum well layers and barrier layers in an InxAlyGa1-x-yN light emitting device typically experience strong internal electric fields even when the device is unbiased.

0006For example, <figref idref="DRAWINGS">FIG. 1</figref> shows a schematic band structure diagram for a portion of an unbiased prior art InxAlyGa1-x-yN LED active region including GaN barrier layer 2, InxGa1-xN quantum well layer 4, and GaN barrier layer 6. The two horizontal axes in <figref idref="DRAWINGS">FIG. 1</figref> represent position in the active region in a direction perpendicular to the layers. The interfaces between the layers are indicated by dashed lines. The lower vertical axis represents the energy of the conduction band edge 8 and of the valence band edge 10 in the various layers. The upper vertical axis represents the concentration of indium in the alloys from which the various layers are formed. Layers 2, 4, and 6 are of wurtzite crystal structure with the c-axis of the crystal substantially perpendicular to the layers and directed from layer 2 toward layer 6. In the prior art active region, the mole fraction of indium is constant across the width of quantum well layer 4.

0007In the absence of a spontaneous polarization, piezoelectric fields, and an externally applied bias, the conduction band edge 8 and valence band edge 10 would be nominally flat within each layer. In the band structure depicted in <figref idref="DRAWINGS">FIG. 1</figref>, however, piezoelectric fields have tilted the band edges. This tilting adversely affects the performance of a light emitting device including the illustrated active region. As a result of this tilting, for example, the electron wave function 12 and the hole wave function 14 are concentrated on opposite sides of InxGa1-xN quantum well 4. The spatial overlap of these wave functions is therefore reduced by the piezoelectric field, leading to a decrease in the probabilities of spontaneous and stimulated emission of light from the active region and an increase in the probability that electrons and holes injected into the active region will relax nonradiatively or leak out of the active region. Hence, the piezoelectric field decreases the operating efficiency of InxAlyGa1-x-yN LEDs and the optical gain in InxAlyGa1-x-yN LDs. Consequently, the piezoelectric field makes high brightness InxAlyGa1-x-yN LEDs and low threshold InxAlyGa1-x-yN LDs difficult to achieve.

0008Another consequence of the piezoelectric field in InxAlyGa1-x-yN light emitting devices is a reduction of the emission energy. Charge injected during operation of the device partially screens the piezoelectric field, however, and results in an increase of the emission energy as the carrier density in the quantum well layer is increased. In high-indium-content quantum well layers this shift can result in drastic color changes with variation in injection current.

0009What is needed is an InxAlyGa1-x-yN light emitting device in which the problems associated with the internal piezoelectric field have been overcome.

SUMMARY

0010A light emitting device in accordance with an embodiment of the present invention includes a first semiconductor layer of a first conductivity type having a first surface, and an active region formed overlying the first semiconductor layer. The active region includes a second semiconductor layer which is either a quantum well layer or a barrier layer. The second semiconductor layer is formed from a III-Nitride semiconductor alloy having a composition graded in a direction substantially perpendicular to the first surface of the first semiconductor layer. The light emitting device also includes a third semiconductor layer of a second conductivity type formed overlying the active region.

0011The second semiconductor layer may be piezoelectric and of, for example, wurtzite crystal structure. In one implementation, a mole fraction of the III-Nitride semiconductor alloy is graded in an asymmetric manner such as, for example, linearly. The composition of the III-Nitride semiconductor alloy may be graded to reduce the effect of a piezoelectric field in the active region. In one implementation, the III-Nitride semiconductor alloy is InxAlyGa1-x-yN and the mole fraction of indium is graded. In another implementation, the III-Nitride semiconductor alloy is InxAlyGa1-x-yN and the mole fraction of aluminum is graded.

0012In one embodiment, the active region includes a plurality of quantum well layers and at least one barrier layer. The barrier layer is formed from a III-Nitride semiconductor alloy having an indium mole fraction graded in a direction substantially perpendicular to the first surface of the first semiconductor layer. In one implementation, the III-Nitride semiconductor alloy is InxAlyGa1-x-yN. The indium mole fraction in the barrier layer may be graded in an asymmetric manner such as linearly, for example, and may be graded to reduce the effect of a piezoelectric field in the active region. The barrier layer may be one of a plurality of barrier layers included in the active region and each formed from a III-Nitride semiconductor alloy having an indium mole fraction graded in a direction substantially perpendicular to the first surface of the first semiconductor layer.

0013Advantageously, the separation of electrons and holes induced by piezoelectric fields in the active regions of prior art light emitting devices is substantially reduced in light emitting devices in accordance with several embodiments of the present invention. Also, in some embodiments the voltage required to drive the light emitting device is reduced. Hence, light emitting devices in accordance with some embodiments of the present invention are more efficient than prior art devices. In addition, in some embodiments the emission wavelength of the light emitting device does not substantially blue shift as the carrier density in the active region is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

0014<figref idref="DRAWINGS">FIG. 1</figref> is a schematic illustration of a band structure for a portion of a prior art InxAlyGa1-x-yN light emitting device.

0015<figref idref="DRAWINGS">FIG. 2</figref> is a schematic diagram of an InxAlyGa1-x-yN LED in accordance with an embodiment of the present invention.

0016<figref idref="DRAWINGS">FIG. 3</figref> is a schematic diagram of the active region of the InxAlyGa1-x-yN LED of <figref idref="DRAWINGS">FIG. 2</figref> in accordance with one embodiment of the present invention.

0017<figref idref="DRAWINGS">FIG. 4</figref> is a schematic illustration of a band structure for a portion of one embodiment of the active region shown in <figref idref="DRAWINGS">FIG. 3</figref>.

0018<figref idref="DRAWINGS">FIG. 5A</figref> is a simulated band structure for a portion of a prior art InxAlyGa1-x-yN light emitting device.

0019<figref idref="DRAWINGS">FIG. 5B</figref> is a simulated band structure for a portion of one embodiment of the active region shown in <figref idref="DRAWINGS">FIG. 3</figref>.

0020<figref idref="DRAWINGS">FIG. 6</figref> is a schematic illustration of a band structure for a portion of another embodiment of the active region shown in <figref idref="DRAWINGS">FIG. 3</figref>.

0021<figref idref="DRAWINGS">FIG. 7</figref> is a simulated band structure for another portion of a prior art InxAlyGa1-x-yN light emitting device.

0022<figref idref="DRAWINGS">FIG. 8</figref> is a schematic diagram of the active region of the InxAlyGa1-x-yN LED of <figref idref="DRAWINGS">FIG. 2</figref> in accordance with another embodiment of the present invention.

0023<figref idref="DRAWINGS">FIG. 9</figref> is a simulated band structure for an embodiment of the InxAlyGa1-x-yN LED of <figref idref="DRAWINGS">FIG. 2</figref> including the active region of <figref idref="DRAWINGS">FIG. 8</figref>.

0024It should be noted that the dimensions in the various figures are not necessarily to scale. Like reference numbers in the various figures denote same parts in the various embodiments.

DETAILED DESCRIPTION

0025In accordance with embodiments of the present invention, the active region of a semiconductor light emitting device includes a semiconductor alloy with a graded composition. Several embodiments will be described in which the active region includes one or more graded composition quantum well layers and/or one or more graded composition barrier layers.

0026Referring to <figref idref="DRAWINGS">FIG. 2</figref>, in one embodiment an InxAlyGa1-x-yN based LED 16 includes a multilayered epitaxial structure 18 disposed on a buffer layer 20, which in turn is disposed on sapphire substrate 22. Epitaxial structure 18 includes active region 24 disposed between p-type upper InxAlyGa1-x-yN region 26 and lower InxAlyGa1-x-yN region 28. InxAlyGa1-x-yN region 28 includes n-type and/or undoped InxAlyGa1-x-yN layers. Active region 24 includes one or more quantum well layers and one or more barrier layers formed from InxAlyGa1-x-yN. Ohmic p-contact 30 and metallic layer 32 are electrically coupled to each other and to upper InxAlyGa1-x-yN region 26. Ohmic n-contact 34 is electrically coupled to lower InxAlyGa1-x-yN region 28. Application of a suitable forward bias across contacts 30 and 34 results in emission of light from active region 24.

0027Referring to <figref idref="DRAWINGS">FIG. 3</figref>, in one embodiment active region 24 includes InxGa1-x-N quantum well layers 36, 40, and 44, and GaN barrier layers 38, 42, and 46, with quantum well layer 36 located closest to sapphire substrate layer 22 (<figref idref="DRAWINGS">FIG. 2</figref>). Quantum well layers 36, 40, and 44 are about 10 Angstroms (Å) to about 100 Å (typically about 30 Å) thick. Barrier layers 38, 42, and 44 are about 25 Å to about 500 Å (typically about 100 Å) thick. Although <figref idref="DRAWINGS">FIG. 3</figref> shows three quantum well layers and three barrier layers, other embodiments include either more or fewer such quantum well and barrier layers.

0028In one embodiment, the mole fraction of indium (subscript x in InxGa1-xN) in one or more of quantum well layers 36, 40, and 44 is graded to decrease with distance from the substrate. For example, the mole fraction of indium in quantum well layer 40 may decrease from a first value near the interface between quantum well layer 40 and barrier layer 38 to a second value near the interface between quantum well layer 40 and barrier layer 42. Typically, the compositions of each of quantum well layers 36, 40, and 44 are similarly graded.

0029The band gap of InxGa1-xN decreases as the mole fraction of indium increases. In the absence of electric fields such as piezoelectric fields, for example, a graded indium concentration which decreases through a quantum well with distance from the substrate results in a graded band gap in the quantum well which increases with distance from the substrate. In such a case the conduction band edge energy in the quantum well would increase with distance from the substrate, and the valence band edge energy in the quantum well would decrease with distance from the substrate. Typically, however, epitaxial structure 18 has a (piezoelectric) wurtzite crystal structure with its c-axis oriented substantially perpendicular to and directed away from sapphire substrate 22. Hence, piezoelectric fields are typically present in quantum wells 36, 40, and 44.

0030Advantageously, a graded indium concentration which decreases through a InxGa1-xN quantum well with distance from the substrate (that is, decreases in a direction substantially parallel to the wurtzite crystal c-axis) at least partially cancels the effect of the piezoelectric field on the conduction band edge in the quantum well. This cancellation can be understood as the result of the tilt of the conduction band edge due to the indium concentration gradient at least partially compensating for the tilt of the conduction band edge due to the piezoelectric fields. One can also understand the effect of the indium concentration gradient on the tilt of the conduction band edge as a partial cancellation of the piezoelectric field by an effective electric field, associated with the composition gradient and opposed to the piezoelectric field, experienced by an electron in the conduction band.

0031The effect of the piezoelectric field on the conduction band edge in a InxGa1-xN quantum well can be substantially eliminated by an indium concentration gradient if the quantum well is sufficiently thin. The magnitude of the piezoelectric field in a InxGa1-xN quantum well layer grown on a GaN layer (for an indium mole fraction x) is approximately EPZ=(7 mega volts per centimeter)·x (this estimate corresponds to a 7 mega volt per centimeter piezoelectric field for an InN layer grown on GaN). The effective electric field experienced by an electron in the conduction band of an InxGa1-xN quantum well of thickness L centimeters and associated with an indium concentration profile that grades in the direction of the wurtzite c-axis from a mole fraction x down to a mole fraction of x=0 is Eeff=(1.05 volts/L)·x. The latter estimate assumes a linear band gap dependence of InxGa1-xN on composition and that about 70% of the 1.5 electron volt (eV) band gap difference between InN and GaN occurs in the conduction band. Equating these electric fields provides a coarse estimate of the width of a linearly graded composition InxGa1-xN quantum well layer for which the effect of the piezoelectric field on the conduction band edge is approximately neutralized: L˜15 Å.

0032<figref idref="DRAWINGS">FIG. 4</figref> shows a schematic band structure of a portion of active region 24 including GaN barrier layers 38 and 42 and a thin InxGa1-xN quantum well layer 40 in an embodiment in which the mole fraction of indium in InxGa1-xN quantum well layer 40 is graded linearly from a maximum value near its interface with barrier layer 38 to about zero near its interface with barrier layer 42. The tilt of conduction band edge 48 in quantum well layer 42 has been substantially reduced compared to the tilt of conduction band edge 8 of prior art quantum well layer 4 in <figref idref="DRAWINGS">FIG. 1</figref>.

0033The tilt of valence band edge 50 in thin quantum well layer 42, however, is similar to or slightly increased with respect to that in prior art quantum well layer 4. The tilt of valence band edge 50 can be understood as the result of the tilt of the valence band edge 50 due to the indium concentration gradient adding to the tilt of the valence band edge 50 due to the piezoelectric fields. One can also understand the effect of the indium concentration gradient on valence band edge 50 as a reinforcement of the piezoelectric field by an effective electric field, associated with the composition gradient, experienced by an electron in the valence band.

0034Advantageously, the separation of electrons and holes that occurs in prior art InxGa1-xN quantum wells is substantially reduced in graded InxGa1-xN quantum wells in accordance with the present invention. In particular, in the embodiment shown in <figref idref="DRAWINGS">FIG. 4</figref> both electrons and holes in quantum well layer 40 concentrate near its interface with barrier layer 38. Hence, light emitting devices in accordance with the present invention are more efficient than prior art devices. In addition, the emission wavelength of a light emitting devices in accordance with the present invention does not substantially blue shift as the carrier density in the quantum well layer is increased.

0035A graded indium concentration which decreases through an InxGa1-xN quantum well layer with distance from the substrate may reduce the separation between electrons and holes in InxGa1-xN even if the quantum well layer is thicker than about 15 Å. For example, <figref idref="DRAWINGS">FIG. 5A</figref> shows a conventionally simulated band structure for a prior art active region including GaN barrier layers 2 and 6 and In0.4Ga0.6N quantum well layer 4 under a forward bias of about 2.5 volts. Barrier layers 2 and 6 are about 100 Å thick. Quantum well layer 4 is about 40 Å thick and has a spatially uniform indium concentration. The conduction band edge 8 and the valence band edge 10 in quantum well layer 4 are tilted in the same manner as in <figref idref="DRAWINGS">FIG. 1</figref>. In contrast, <figref idref="DRAWINGS">FIG. 5B</figref> shows a simulated band structure for active region 24 (under a forward bias of about 2.5 volts) in which GaN barrier layers 38 and 42 are about 100 Å thick, quantum well layer 40 is about 40 Å thick, and the mole fraction of indium in InxGa1-xN quantum well layer 40 grades linearly from a mole fraction of about x=0.4 near its interface with barrier layer 38 to a mole fraction of about x=0 near its interface with barrier layer 42. Conduction band edge 48 has been flattened, compared to conduction band edge 8 in <figref idref="DRAWINGS">FIG. 5A</figref>. As in the embodiment shown in <figref idref="DRAWINGS">FIG. 4</figref>, both electrons and holes in quantum well layer 40 concentrate near its interface with barrier layer 38. Hence, this embodiment also achieves the advantages described above with respect to the embodiment shown in <figref idref="DRAWINGS">FIG. 4</figref>.

0036In another embodiment, the mole fraction of indium in one or more of quantum well layers 36, 40, and 44 is graded to increase with distance from the substrate. For example, the mole fraction of indium in quantum well layer 40 may increase from a first value near the interface between quantum well layer 40 and barrier layer 38 to a second value near the interface between quantum well layer 40 and barrier layer 42. Advantageously, a graded indium concentration which increases through an InxGa1-xN quantum well in a direction substantially parallel to the wurtzite crystal c-axis at least partially cancels the effect of the piezoelectric field on the valence band edge in the quantum well. This cancellation can be understood similarly to the cancellation of the effect of the piezoelectric field on the conduction band edge described above.

0037<figref idref="DRAWINGS">FIG. 6</figref> shows a schematic band structure of a portion of active region 24 including GaN barrier layers 38 and 42 and a thin InxGa1-xN quantum well layer 40 in an embodiment in which the mole fraction of indium in InxGa1-xN quantum well layer 40 is graded linearly from a value of about zero near its interface with barrier layer 38 to a maximum value near its interface with barrier layer 42. The tilt of valence band edge 50 in quantum well layer 42 has been substantially reduced compared to the tilt of valence band edge 10 of prior art quantum well layer 4 in <figref idref="DRAWINGS">FIG. 1</figref>. The tilt of the conduction band edge in this embodiment can be understood similarly to the tilt of the valence band edge in the embodiment shown in <figref idref="DRAWINGS">FIG. 4</figref>.

0038The separation of electrons and holes that occurs in prior art InxGa1-xN quantum wells is substantially reduced in this embodiment as well. In particular, in this embodiment both electrons and holes in quantum well layer 40 tend to concentrate near its interface with barrier layer 42. Hence, this embodiment also achieves the advantages described above with respect to the embodiments shown in <figref idref="DRAWINGS">FIGS. 4 and 5B</figref>.

0039The above embodiments demonstrate that it can be advantageous to grade the indium concentration in an InxGa1-xN quantum well to either increase or decrease in a direction substantially parallel to the wurtzite crystal c-axis. In active regions in which the offset of the conduction band edge in the quantum well and barrier layers is larger than the offset of the valence band edge, it is typically more advantageous to grade the indium concentration to decrease in the direction of the c-axis. In active regions in which the offset of the conduction band edge in the quantum well and barrier layers is smaller than the offset of the valence band edge, it is typically more advantageous to grade the indium concentration to increase in the direction of the c-axis.

0040Although the mole fraction of indium in quantum well layer 40 is graded linearly in the embodiments shown in <figref idref="DRAWINGS">FIGS. 4</figref>, 5B, and 6, other functional forms for the position dependence of the indium mole fraction in one or more of the quantum well layers in active region 24 may also be used. For example, the indium mole fraction may grade in an exponential, parabolic, or step-wise manner. Also, although the indium mole fraction decreases or increases monotonically across a quantum well in the embodiments shown in <figref idref="DRAWINGS">FIGS. 4</figref>, 5B, and 6, the mole fraction of indium may instead have a global maximum and/or one or more local maxima at one or more intermediate positions in the quantum well. Typically, the position dependence of the mole fraction of indium in an InxGa1-xN quantum well in accordance with an embodiment of the present invention is asymmetric with respect to a plane parallel to the barrier layers and about centered in the quantum well.

0041The mole fraction of indium in a graded InxGa1-xN quantum well in accordance with an embodiment of the present invention may range, for example, from about x=0.5 to about x=0. The mole fraction of indium in such a graded quantum well may be greater than zero at both of the quantum well's interfaces with barrier layers. That is, the mole fraction of indium need not grade all the way down to x=0.

0042The tilt of the conduction band edge in the barrier layers of the active region of an InxAlyGa1-x-yN light emitting device also adversely affects the performance of the device. <figref idref="DRAWINGS">FIG. 7</figref> shows a conventionally simulated conduction band edge 52 for a portion of a prior art InxAlyGa1-x-yN light emitting device including GaN layer 54, GaN barrier layers 58, 62, and 66, and In0.4Ga0.6N quantum well layers 56, 60, and 64 under a forward bias of about 2.5 volts. The tilt of conduction band edge 52 in GaN layers 54, 58, 62, and 66 produces triangular potential barriers 68, 70, and 72. Electrons must be injected across these potential barriers, resulting in a disadvantageous increase of the diode voltage.

0043The tilt of the conduction band edge in the barrier layers may be reduced by grading the composition of one or more of the barrier layers. Referring to <figref idref="DRAWINGS">FIG. 8</figref>, for example, in one embodiment active region 24 of LED 16 (<figref idref="DRAWINGS">FIG. 2</figref>) includes Inx2Ga1-x2N quantum well layers 74, 78, and 82, and Inx1Ga1-x1N barrier layers 76, 80, and 84, with quantum well layer 74 located closest to sapphire substrate layer 22 (<figref idref="DRAWINGS">FIG. 2</figref>). The mole fraction x2 of indium in quantum well layers 74, 78, and 82 is spatially uniform. The mole fraction x1 of indium in one or more of barrier layers 76, 80, and 84, however, grades with distance from substrate 22. The mole fraction of indium in a barrier layer may be graded, for example, to increase or to decrease with distance from substrate 22. The functional form for the position dependence of the indium mole fraction in a barrier layer may be, for example, linear, exponential, parabolic, or step-wise. The mole fraction of indium in a barrier layer may have a global maximum and/or one or more local maxima at one or more intermediate positions in the barrier layer. Typically, the position dependence of the mole fraction of indium in a barrier layer is asymmetric.

0044One of ordinary skill in the art would expect that introduction of indium into a III-Nitride barrier layer would adversely affect device performance by decreasing carrier confinement. The inventors have discovered, however, that grading the indium mole fraction in an InxGa1-xN barrier layer is beneficial. For example, <figref idref="DRAWINGS">FIG. 9</figref> shows conventionally simulated conduction band edge 48 for a portion of LED 16 including active region 24, in accordance with this embodiment, under a forward bias of about 2.5 volts. In the implementation illustrated in <figref idref="DRAWINGS">FIG. 9</figref>, the composition of quantum well layers 74, 78, and 82 is In0.4Ga0.6N, and the mole fraction of indium in each of barrier layers 76, 80, and 84 grades from about x=0 near one side of each barrier layer to about x=0.16 near the other side of each barrier layer in the direction of the wurtzite c-axis (bottom to top in <figref idref="DRAWINGS">FIG. 8</figref>). Advantageously, the height of the triangular potential barriers to electron injection into quantum well layers 74, 78, and 82 are substantially reduced compared to barriers 68, 70, and 72 of <figref idref="DRAWINGS">FIG. 7</figref>. In other implementations, the mole fraction of indium in the quantum well layers may be greater than or less than x2=0.4 and the mole fraction of indium in the barrier layers may be greater than 0.16.

0045In other embodiments, active region 24 includes one or more graded composition quantum well layers and one or more graded composition barrier layers. Also, in other embodiments the mole fraction of aluminum in an InxAlyGa1-x-yN quantum well layer or barrier layer is graded in a direction substantially perpendicular to the layers. In some embodiments, both the mole fraction of aluminum and the mole fraction of indium are graded.

0046The various InxAlyGa1-x-yN layers in a light emitting device in accordance with an embodiment of the present invention may be formed by, for example, metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Graded composition quantum well and barrier layers may be formed, for example, by varying the flow rates of reagent gases during layer deposition.

0047While the present invention is illustrated with particular embodiments, the invention is intended to include all variations and modifications falling within the scope of the appended claims. Referring again to <figref idref="DRAWINGS">FIG. 2</figref>, for example, substrate 22 may be formed from materials other than sapphire, such as SiC. Metallic layer 32 may be semitransparent to light emitted by active region 24. Alternatively, metallic layer 32 may be highly reflective to light emitted by active region 24, and LED 16 may be mounted as a flip-chip with contacts 30 and 34 facing a submount.

0048Graded composition active regions in accordance with the present invention may be formed in other InxAlyGa1-x-yN light emitting devices such as, for example, those described in U.S. Pat. No. 6,133,589, assigned to the assignee of the present invention and incorporated herein by reference in its entirety. Moreover, an InxAlyGa1-x-yN light emitting device in accordance with the present invention may have, in contrast to LED 16 of <figref idref="DRAWINGS">FIG. 2</figref>, a structure in which an n-type region overlies a p-type region, which overlies a substrate. The light emitting device may be a laser diode.

0049Graded composition active regions in accordance with the present invention may also be formed in other material systems such as other III-V material systems and II-VI material systems. Such graded active regions can be particularly advantageous in piezoelectric material systems and in material systems having a spontaneous polarization.

Contents5

Every citation, both ways

Citations
Document	Relation	Office	Cited during
US8987755B1	Cited by	United States of America	Search report
US9263632B2	Cited by	United States of America	Applicant
US9627580B2	Cited by	United States of America	Applicant
US8212262B2	Cited by	United States of America	Search report
US2014008609A1	Cited by	United States of America	Pre-grant
US10535801B2	Cited by	United States of America	Applicant
US11502220B1	Cited by	United States of America	Applicant
US11646395B2	Cited by	United States of America	Applicant
US2013087761A1	Cited by	United States of America	Pre-grant
US8227791B2	Cited by	United States of America	Applicant
US10593830B1	Cited by	United States of America	Applicant
US2008191224A1	Cited by	United States of America	Pre-grant
KR101717623B1	Cited by	Republic of Korea	Applicant
US9318652B1	Cited by	United States of America	Applicant
US9048362B2	Cited by	United States of America	Search report
EP2348548A2	Cited by	European Patent Office (EPO)	Applicant
US2010187496A1	Cited by	United States of America	Pre-grant
EP2348549A2	Cited by	European Patent Office (EPO)	Applicant
US9780254B1	Cited by	United States of America	Applicant
WO0141224A2	Cites	World Intellectual Property Organization (WIPO)	Applicant
US5670798A	Cites	United States of America	Applicant
US5810925A	Cites	United States of America	Applicant
US5960018A	Cites	United States of America	Applicant
US6045626A	Cites	United States of America	Applicant
US6100106A	Cites	United States of America	Search report
US6133589A	Cites	United States of America	Applicant
US6262465B1	Cites	United States of America	Search report
US6515313B1	Cites	United States of America	Applicant
US6630692B2	Cites	United States of America	Applicant
US6995389B2	Cites	United States of America	Search report
US7122839B2	Cites	United States of America	Search report
WO0141224A2	Cites	World Intellectual Property Organization (WIPO)	Third party observation
"Physics of Semiconductor Devices" 2nd Edition, by S. M. Sze, p. 9.	Non-patent	–	Search report
Andreas Hangleiter et al., "The Role Of Piezoelectric Fields In GaN-Based Quantum Wells," MRS Internet J. Nitride Semiconductor Research, 3, 15 (1998), 1998-1999 The Materials Research Society, pp. 1-8.	Non-patent	–	Applicant
Fabio Bernardini et al., "Spontaneous Polarization And Piezoelectric Constants Of III-V Nitrides," vol. 56, No. 16, Oct. 15, 1997, The American Physical Society, 4 pages.	Non-patent	–	Applicant
Tetsuya Takeuchi et al,"Determination Of Piezoelectric Fields In Strained GaInN Quantum Wells Using The Quantum-Confined Stark Effect," Applied Physics Letters, vol. 73, No. 12, Sep. 21, 1998, American Institute of Physics, pp. 1691-1693.	Non-patent	–	Applicant
S.F. Chichibu et al., "Optical Properties Of InGaN Quantum Wells," Materials Science and Engineering B59 (1999) pp. 298-306.	Non-patent	–	Applicant
S.F. Chichibu et al., "Effective Band Gap Inhomogeneity And Piezoelectric Field In InGaN/GaN Multiquantum Well Structures," Applied Physics Letters, vol. 73, No. 14, Oct. 5, 1998, American Institute of Physics, pp. 2006-2008.	Non-patent	–	Applicant
Takashi Mukai, "Current And Temperature Dependences Of Electroluminescence Of InGaN-Based UV/Blue/Green Light-Emitting Diodes," Japanese Journal of Applied Physics, vol. 37 (1998) Pt. 2, No. 11B, pp. L1358-L1361.	Non-patent	–	Applicant
Fabio Della Sala et al., "Free-Carrier Screening Of Polarization Fields In Wurtzite GaN/InGaN Laser Structures," Applied Physics Letters, vol. 74, No. 14, Apr. 5, 1999, American Institute of Physics, pp. 2002-2004.	Non-patent	–	Applicant
L. H. Peng et al., "Piezoelectric Effects In The Optical Properties Of Strained InGaN Quantum Wells," Applied Physics Letters, vol. 74, No. 6, Feb. 8, 1999, American Institute of Physics, pp. 795-797.	Non-patent	–	Applicant
W.W. Chow et al., "Quantum-Well Width Dependence Of Threshold Current Density In InGaN Lasers," vol.75, No. 2, Jul. 12, 1999, American Institute of Physics, pp. 244-246.	Non-patent	–	Applicant
Shuji Nakamuira et al., "Introduction to Nitride Semiconductor Blue Lasers and Light Emitting Diodes," ISBN 0-7484-0836-3, 8 book pages.	Non-patent	–	Applicant
Tetsuya Takeuchi et al., "Theoretical Study of Orientation Dependence of Piezoelectric Effects in Wurtzite Strained GaInN/GaN Heterostructures and Quantum Wells," Publication Board, Japanese Journal of Applied Physics, vol. 39, Part 1, No. 2A, Feb. 2000, pp. 413-416.	Non-patent	–	Applicant
Tetsuya Takeuchi et al., "Quantum-Confined Stark Effect Due to Piezoelectric Fields in GaInN Strained Quantum Wells," Japanese Journal of Applied Physics, vol. 36, Part 2, No. 4A, Apr. 1, 1997, pp. L382-385.	Non-patent	–	Applicant
K. Horino et al., "Initial Growth Stage of AIGaN Grown Directly on (0001) 6H-SiC By MOVPE," Mat. Res. Soc. Symp. Proc. vol. 449, 1997 Materials Research Society, pp. 73-78.	Non-patent	–	Applicant
Dongjin Byun et al., Optimization of the GaN-Buffer Growth on 6H-SiC (0001), Thin Solid Films 289 (1996) pp. 256-260.	Non-patent	–	Applicant
Noriyuki Kiwano et al., Cross-Sectional TEM Study Of Microstructures in MOVPE GaN films Grown On a-AI203 With A Buffer Layer Of AIN, Journal of Crystal Growth 115 (1991), pp. 381-387.	Non-patent	–	Applicant
Shuji Nakamura, "GaN Growth Using GaN Buffer Layer," Japanese Journal of Applied Physics, vol. 30, No. 10A, Oct. 1991, pp. L1705-L1707.	Non-patent	–	Applicant
H. Amano et al., "Metalorganic Vapor Phase Epitaxial Growth Of A High Quality GaN Film Using An AIN Buffer Layer," Applied Physics Letter, vol. 48, No. 5, Feb. 3, 1986, pp. 353-355.	Non-patent	–	Applicant
“Physics of Semiconductor Devices” 2nd Edition, by S. M. Sze, p. 9.	Non-patent	–	Search report
Andreas Hangleiter et al., “The Role Of Piezoelectric Fields In GaN-Based Quantum Wells,” MRS Internet J. Nitride Semiconductor Research, 3, 15 (1998), 1998-1999 The Materials Research Society, pp. 1-8.	Non-patent	–	Third party observation
Fabio Bernardini et al., “Spontaneous Polarization And Piezoelectric Constants Of III-V Nitrides,” vol. 56, No. 16, Oct. 15, 1997, The American Physical Society, 4 pages.	Non-patent	–	Third party observation
Tetsuya Takeuchi et al,“Determination Of Piezoelectric Fields In Strained GaInN Quantum Wells Using The Quantum-Confined Stark Effect,” Applied Physics Letters, vol. 73, No. 12, Sep. 21, 1998, American Institute of Physics, pp. 1691-1693.	Non-patent	–	Third party observation
S.F. Chichibu et al., “Optical Properties Of InGaN Quantum Wells,” Materials Science and Engineering B59 (1999) pp. 298-306.	Non-patent	–	Third party observation
S.F. Chichibu et al., “Effective Band Gap Inhomogeneity And Piezoelectric Field In InGaN/GaN Multiquantum Well Structures,” Applied Physics Letters, vol. 73, No. 14, Oct. 5, 1998, American Institute of Physics, pp. 2006-2008.	Non-patent	–	Third party observation
Takashi Mukai, “Current And Temperature Dependences Of Electroluminescence Of InGaN-Based UV/Blue/Green Light-Emitting Diodes,” Japanese Journal of Applied Physics, vol. 37 (1998) Pt. 2, No. 11B, pp. L1358-L1361.	Non-patent	–	Third party observation
Fabio Della Sala et al., “Free-Carrier Screening Of Polarization Fields In Wurtzite GaN/InGaN Laser Structures,” Applied Physics Letters, vol. 74, No. 14, Apr. 5, 1999, American Institute of Physics, pp. 2002-2004.	Non-patent	–	Third party observation
L. H. Peng et al., “Piezoelectric Effects In The Optical Properties Of Strained InGaN Quantum Wells,” Applied Physics Letters, vol. 74, No. 6, Feb. 8, 1999, American Institute of Physics, pp. 795-797.	Non-patent	–	Third party observation
W.W. Chow et al., “Quantum-Well Width Dependence Of Threshold Current Density In InGaN Lasers,” vol.75, No. 2, Jul. 12, 1999, American Institute of Physics, pp. 244-246.	Non-patent	–	Third party observation
Shuji Nakamuira et al., “Introduction to Nitride Semiconductor Blue Lasers and Light Emitting Diodes,” ISBN 0-7484-0836-3, 8 book pages.	Non-patent	–	Third party observation
Tetsuya Takeuchi et al., “Theoretical Study of Orientation Dependence of Piezoelectric Effects in Wurtzite Strained GaInN/GaN Heterostructures and Quantum Wells,” Publication Board, Japanese Journal of Applied Physics, vol. 39, Part 1, No. 2A, Feb. 2000, pp. 413-416.	Non-patent	–	Third party observation
Tetsuya Takeuchi et al., “Quantum-Confined Stark Effect Due to Piezoelectric Fields in GaInN Strained Quantum Wells,” Japanese Journal of Applied Physics, vol. 36, Part 2, No. 4A, Apr. 1, 1997, pp. L382-385.	Non-patent	–	Third party observation
K. Horino et al., “Initial Growth Stage of AIGaN Grown Directly on (0001) 6H—SiC By MOVPE,” Mat. Res. Soc. Symp. Proc. vol. 449, 1997 Materials Research Society, pp. 73-78.	Non-patent	–	Third party observation
Dongjin Byun et al., Optimization of the GaN-Buffer Growth on 6H—SiC (0001), Thin Solid Films 289 (1996) pp. 256-260.	Non-patent	–	Third party observation
Noriyuki Kiwano et al., Cross-Sectional TEM Study Of Microstructures in MOVPE GaN films Grown On a-AI203 With A Buffer Layer Of AIN, Journal of Crystal Growth 115 (1991), pp. 381-387.	Non-patent	–	Third party observation
Shuji Nakamura, “GaN Growth Using GaN Buffer Layer,” Japanese Journal of Applied Physics, vol. 30, No. 10A, Oct. 1991, pp. L1705-L1707.	Non-patent	–	Third party observation
H. Amano et al., “Metalorganic Vapor Phase Epitaxial Growth Of A High Quality GaN Film Using An AIN Buffer Layer,” Applied Physics Letter, vol. 48, No. 5, Feb. 3, 1986, pp. 353-355.	Non-patent	–	Third party observation

7 members in 4 offices

This recordMember, by publication datePriority claim

Priority claims6

Priority claims
Document	Office	Kind	Date
91258901	United States of America	A	2001-07-24
91258901	United States of America	A	2001-07-24
17767005	United States of America	A	2005-07-07
09912589	–	–	–
US20010912589	–	–	–
US20050177670	–	–	–

Members7

Family members, oldest first
Document	Office	Kind
EP1280212A2	European Patent Office (EPO)	A2	2003-01-29
US2003020085A1	United States of America	A1	2003-01-30
JP2003060232A	Japan	A	2003-02-28
TW556361B	Taiwan Province of China	B	2003-10-01
US6955933B2	United States of America	B2	2005-10-18
US2005263780A1	United States of America	A1	2005-12-01
US7345324B2This record	United States of America	B2	2008-03-18

35 transactions on the USPTO file

Allowed after 2 non-final rejections.

Non-final rejections: 2
Final rejections: 0
RCEs: 0
Appeals: 0

Over time

Point at a mark for the transaction

Transactions

Prosecution transactions, newest first
	Event	Code
2012-04-16	Expire PatentEXP.	EXP.
2008-03-18	Recordation of Patent Grant MailedPGM/	PGM/
2008-03-18	Patent Issue Date Used in PTA CalculationAllowedPTAC	PTAC
2008-02-27	Issue Notification MailedAllowedWPIR	WPIR
2008-02-08	Dispatch to FDCD1935	D1935
2008-02-08	Application Is Considered Ready for IssuePILS	PILS
2008-01-18	Issue Fee Payment VerifiedN084	N084
2008-01-18	Issue Fee Payment ReceivedIFEE	IFEE
2007-10-23	Mail Notice of AllowanceAllowedMN/=.	MN/=.
2007-10-22	Notice of Allowance Data Verification CompletedAllowedN/=.	N/=.
2007-10-22	Case Docketed to Examiner in GAUDOCK	DOCK
2007-10-10	Paralegal or electronic terminal disclaimer approvedP574	P574
2007-10-10	Date Forwarded to ExaminerFWDX	FWDX
2007-10-02	Terminal Disclaimer FiledDIST	DIST
2007-10-02	Response after Non-Final ActionA...	A...
2007-10-02	Request for Extension of Time - GrantedXT/G	XT/G
2007-09-14	Mail Examiner Interview Summary (PTOL - 413)MEXIN	MEXIN
2007-09-13	Examiner Interview Summary Record (PTOL - 413)EXIN	EXIN
2007-06-11	Mail Non-Final RejectionNon-final rejectionMCTNF	MCTNF
2007-06-07	Non-Final RejectionNon-final rejectionCTNF	CTNF
2007-04-05	Date Forwarded to ExaminerFWDX	FWDX
2007-03-23	Response after Non-Final ActionA...	A...
2007-03-23	Request for Extension of Time - GrantedXT/G	XT/G
2006-12-20	Mail Non-Final RejectionNon-final rejectionMCTNF	MCTNF
2006-12-15	Non-Final RejectionNon-final rejectionCTNF	CTNF
2006-02-10	IFW TSS Processing by Tech Center CompleteTSSCOMP	TSSCOMP
2006-02-10	Case Docketed to Examiner in GAUDOCK	DOCK
2005-08-20	Cleared by L&R (LARS)L128	L128
2005-08-15	Application Dispatched from OIPEOIPE	OIPE
2005-08-15	Application Is Now CompleteCOMP	COMP
2005-07-25	Referred to Level 2 (LARS) by OIPE CSRL198	L198
2005-07-21	IFW Scan & PACR Auto Security ReviewSCAN	SCAN
2005-07-07	Reference capture on IDSRCAP	RCAP
2005-07-07	Preliminary AmendmentA.PE	A.PE
2005-07-07	Initial Exam Team nnIEXX	IEXX

7 legal events, as the office reported them to INPADOC

Over the term

Point at a mark for the event

Events

Legal events, newest first
	Event	Code
2018-04-16	AssignmentAS	AS
2018-04-13	AssignmentAS	AS
2018-04-09	AssignmentAS	AS
2012-05-08	Lapsed due to failure to pay maintenance feeLapsedFP	FP
2012-04-16	Information on status: patent discontinuationPATENT EXPIRED DUE TO NONPAYMENT OF MAINTENANCE FEES UNDER 37 CFR 1.362STCH	STCH
2012-03-18	Lapse for failure to pay maintenance feesLapsedLAPS	LAPS
2011-10-31	Maintenance fee reminder mailedREMI	REMI